Introduction

This post originated from my personal notes while learning Linux back when there was no AI assistance, so you’ll find a mix of beginner and more advanced commands. All of them work, and remember that in Linux there are many ways to achieve the same task.

Also remember to search using CTRL+F, both with accents and without accents, because sometimes I typed with an English keyboard and it wasn’t easy to use accent marks.
For example, if you want to see how to set a static IP in CentOS, search for “direccion” without an accent and jump through the results until you find what you’re looking for.

These commands are mainly from Debian and RedHat families.

Index

• 01 - General Commands
• 02 - File Viewing, Search & Text Processing
• 03 - File & Directory Management
• 04 - Users, Groups & Permissions
• 05 - Networking & IP Configuration
• 06 - SSH, SCP & Remote Access
• 07 - Processes & System Monitoring
• 08 - Package Management (RPM, YUM, DPKG, APT)
• 09 - Firewall / IPTables
• 10 - VNC, X11 & GUI Tools
• 11 - Crontab & Automation
• 12 - Disk, Devices & System Boot
• 13 - Miscellaneous Utilities
• 14 - Keyboard Shortcuts

General commands

– Requesting Help for Any Command

man <comando>

Example:
man grep

– Disk Partitioning

– Create, View, Delete Partitions

fdisk /dev/hda

– Create a Partition on the Primary Hard Drive

mkfs -t ext2 -c /dev/hda1

• -t specifies the filesystem type (ext2 in this case).
• -c checks for bad sectors before formatting.

Recommended disk layout (varies depending on intended Linux use):

• /boot → 102–150 MB
• /swap → Twice the physical memory
• / (root) → The rest of the disk
• Ext3 is preferable → ext2 with journaling

– Clear the Screen

clear

– Reset Terminal to Default Values

reset

– Show System Users Information

finger

– Show Users Currently Logged In

who

– Display Logged‑in Users and Their Sessions/Tasks

w

– Show Current Username

whoami

– Show Numeric IDs and Groups for Current User

id

– Show Current Date and Time

date

– Show Calendar for Current Month

cal

– View File Contents

cat

– Exit the Current Shell

exit

– Print Output to the Screen

echo

– Show Current Shell

echo $SHELL

– Shell Variable

$

– Home Directory Shortcut

Used for example with cd ~ which takes you to /home/<user>.

~

– Run a Command in the Background

Example:
nmap -O 192.168.1.1-24 & runs Nmap in the background while you continue using the terminal.

&

– Input/Output Redirection

< << > >>

– Pipe to Chain Commands

Example:
cat file.txt | grep name

|

– Shell Wildcards

Example:
ls *.txt

?[]*

– Quoting and Metacharacters

' " \

– Command Substitution

`

– Grouping Commands

() {}

– Cancel a Running Command

Press and hold CTRL, then press C.

CTRL+C

File Viewing, Search & Text Processing

– View the Last 10 Lines of a Text File

tail <nombrearchivo>

– View the Last 30 Lines

tail -30 <nombrearchivo>

– Follow File Changes in Real Time (useful for logs)

tail -f <nombredelarchivo>

– View the First 10 Lines

head <nombrearchivo>

– View the First 30 Lines

head -30 <nombrearchivo>

– List the Most Recently Modified File in a Directory

ls -Art | tail -n 1

– Show Only Directories

ls -l|grep ^d
ls -p |grep /
ls -l | egrep '^d'

– Search for Text in Files (basic examples)

grep "spc_eee1.gif" *
grep -r -i "textoAbuscar" *.*

-r = recursive
-i = ignore case

– Search for Text and Show Line Numbers

grep -nr textoabuscar

– Use awk to Extract Columns (example: extract IPs from Apache logs)

awk '{print $1}' access-hackvolution.log

– Remove Duplicate Results Using sort and uniq

awk '{print $1}' access-hackvolution.log |sort |uniq

– Count Repeated Values and Sort by Frequency

awk '{print $1}' access-hackvolution.log |sort |uniq -c |sort -rn

– Search Inside .sh Scripts for the Word “user”

find / -name '*.sh' 2>/dev/null | xargs grep -i 'user'

– Search for Script Files (.ksh, .sh, .pl, .py, .zsh, .bash, .bsh, .sql)

find / -type f \( -iname \*.ksh -o -iname \*.sh -o -iname \*.pl -o -iname \*.py -o -iname \*.zsh -o iname \*.bash -o -iname \*.bsh -o -iname \*.sql \)

– Search for Files Owned by a User and Starting with “#!” (shebang)

find / -user $UID -type f | while read fn; do head -n1 "$fn" | grep -q "^#\!" && echo "$fn" && chmod 755 "$fn"; done

– Search for the Word “password” in Script Files

grep --include=\*.{ksh,sh,pl,py,zsh,bash,bsh,sql} -rnw '/' -e "password"

– Alternative Text Search Examples

grep 'user' $(find / -name '*.sh')
find / -name '*.sh' -exec grep 'texto' {} \;

– Remove Commented Lines from squid.conf (remove lines starting with # and empty lines)

cat /etc/squid/squid.conf | egrep -v "^\s*(#|$)"

– Remove First and Last Character of Each Line

cat lista.txt | cut -c 2- | sed 's/.$//' > clean-lista.txt

– Extract Valid IPs Using Regex

grep -oE '(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)' file.txt

– Search for Files Containing “wants to contact you”

find ./ -name *.htm |xargs grep "wants to contact you"

– Exclude a Directory While Searching

find / \( ! -name tmp -o -prune \) -name "file.rar" -print 2>/dev/null

– Search Using locate (requires index)

locate <nombredelarchivoabuscar>

– Remove Comments and Clean Configuration Files Using sed

sed -e '/string-aqui/d' /opt/archivo-origen.csv > archivodestino.csv

– Remove Windows CR Line Endings

sed -i 's/\r$//' nombredelarchivo.txt

– Delete Everything From a Keyword to End of File

sed -i '/mfa/,$d' ~/.aws/credentials

– Convert DOS Line Endings to Unix Using dos2unix

dos2unix <nombredelarchivo>

– Hexadecimal View of Files

hexdump -C <nombrearchivo>

– Sort IPs Numerically

sort -t . -k 1,1n -k 2,2n -k 3,3n -k 4,4n ips.txt

– Quick Version-Based Sort

sort -V ips.txt

– Extract Public IP

dig +short myip.opendns.com @resolver1.opendns.com

– Convert File Names from Windows Encoding to UTF‑8

convmv -f latin1 -t utf-8 -r --notest *

– Convert ANSI/ASCII Files to UTF‑8

iconv -f 8859_1 -t utf-8 2lista.lst > lista.lst

File & Directory Management

– Clear the Screen

clear

– Exit the Current Shell

exit

– Print Text on Screen

echo

– List Files in Long Format

ls -l

– Show Number of Files in a Directory

echo *.* | wc

– Show Number of Directories in a Folder

echo */ | wc

– Delete All Files with Extension *.xxx Across the Entire Disk

find / -type f -name "*.xxx" -exec rm {} \;

– Delete Directories (Be Careful With -rf)

rm -rf foldername

– Remove Empty Directories

rmdir <nombredelfolder>

– Delete All Files Except One Folder (example: keep “wiki”)

shopt -s extglob
rm -rf -v !("wiki")

– Copy Entire CD-ROM Contents to a Folder

cp -av /mnt/cdrom /install

– Copy Files Incrementally Using rsync

rsync -avzhP /home/lmoreno/datos lmoreno@192.168.1.50:/mnt/data/datosbackup

-v verbose
-r recursive
-a archive mode
-z compress
-h human readable
-P progress

– Copy Remote Directory to Local Directory

rsync -aOvzhP -e "ssh -i EC2.private" root@192.168.1.30:/var/www/html/content/user_1/shoutcast198 /home/ec2-user/logs/

-O do not copy folder timestamps

– Move Many Files When “Argument List Too Long” Error Appears

find . -type f -name "*.txt" -exec mv {} /opt/movehere/ \;

– Move Files One by One Using find

Example of workaround for “Argument list too long”:

-bash: /bin/mv: Argument list too long

for i in `find . -name ap\*`
do
mv $i ..
done

– Change File Extensions in Bulk

for f in *.htm; do mv $f `basename $f .htm`.html; done;

– Concatenate Files into One

cat *.csv >> archivoconcatenado.csv

– Create Backup of a Folder in Compressed Form

tar -czvf myarchive.tgz mydirectory/

– Compress a Folder into a .tar.gz File

tar -zcvf archive_name.tar.gz directory_to_compress

-z gzip
-c create
-v verbose
-f file

– Compress Entire Folder in zip Format

zip -qq -R archivozip directorio

– Compress All .html Files into a Zip

find . -name "*.html" | xargs zip HTMLs-Nessus

– Extract a .zip File

unzip nombre-archivo

– Extract a tar.bz2 File

tar -xvjf file.tar.bz2

– Extract a .tar File

tar -xvf foo.tar

– Extract a .gz File

gunzip archivo.gz

– Extract Contents of a .zip Without Output

unzip -qq archive -d ruta-directorio

– Extract rar Files (install unrar first)

yum install unrar

Then:

unrar x archivo.rar

– Split a File Into Smaller Parts (by size example)

split -b 200M nombredearchivo

– Sort Folder Sizes from Smallest to Largest

du -hs * | sort -h

– Show Largest Files in Current Directory (top 10)

du -hs * | sort -rh | head -10

– Show Largest Files Including Subdirectories (top 10)

du -Sh | sort -rh | head -10

– Show Used Disk Space

df -h

– Show Size of a Specific Folder

du -s /media/Nabucodonosor/data

– Show Disk Usage Summary of Subfolders

du --max-depth=1 -h

– Create a File Filled with Zeros

dd if=/dev/zero of=/test bs=1048576 count=1024

Users, Groups & Permissions

– Add a User

useradd <nombreusuario>

– Set User Password

passwd <nombreusuario>

Example:

useradd -c "Luis Moreno" morenol
passwd mi_password

– Add User With Custom Home and Shell

/usr/sbin/useradd -d /home/lmoreno -s /bin/bash -c "Luis Moreno" lmoreno && mkdir /home/lmoreno && chown lmoreno /home/lmoreno && passwd mysupapassw0rd

– Same as Above but Without Setting Password

/usr/sbin/useradd -d /home/lmoreno -s /bin/bash -c "Luis Moreno" lmoreno && mkdir /home/lmoreno && chown lmoreno /home/lmoreno

– Delete a User

deluser <nombredeusuario>

Example:
deluser -r dsantos
-r removes home directory and mailbox

– List System Users

cat /etc/passwd

– List System Groups

cat /etc/group

– Add a Group

groupadd <nombredelgrupo>

– Lock/Disable a User (manual method)

agregar el signo "+" delante de la línea del usuario en el archivo /etc/passwd

– Lock User Using passwd

passwd -l <nombreUsuario>

– Add User to Supplementary Group

usermod -a -G web luis

– Change User’s Primary Group

usermod -g www luis

– Change File Permissions (symbolic)

chmod u+wr,g+wr,o-r

– Change File Permissions (numeric)

chmod 664

read = 4
write = 2
execute = 1

– Make File Executable for Everyone

chmod 111 <nombreDelArchivo>

– Give Root Full Permissions on a File

chmod 700 <nombreDelArchivo>

– Give Full Permissions to Everyone (dangerous)

chmod 777 <nombreDelArchivo>

– Apply rwx Permissions Recursively on Folder and Subfolders

chmod 777 -R archivos/*

– Clone Permissions From One File to Another

chown --reference=otherfile thisfile

– Create a Device File

mknod /dev/fd1 b 2 1

– Change File Owner

chown luis /carpeta/archivo

– Change File Owner and Group

chown luis.grupo /carpeta/archivo

– Change Group Ownership Only

chgrp grupo /carpeta/archivo

– Password Policy Configuration: Edit /etc/login.defs

PASS_MAX_DAYS 60
PASS_MIN_DAYS 0
PASS_MIN_LEN 8
PASS_WARN_AGE 10

Explanation:

• PASS_MAX_DAYS → maximum days before requiring password change
• PASS_MIN_DAYS → minimum days before password can be changed again
• PASS_MIN_LEN → minimum password length
• PASS_WARN_AGE → expiration warning days

– Apply Password Policy to Existing Users Using chage

– Enable Strong Password Enforcement With pam_cracklib

Networking & IP Configuration

– Show Network Configuration (CentOS/RedHat)

cat /etc/sysconfig/network-scripts/ifcfg-eth0

– Show Network Configuration (Debian/Ubuntu)

cat /etc/sysconfig/network

– Example Network Configuration (CentOS)

HWADDR=00:08:A2:0A:BA:B8
TYPE=Ethernet
BOOTPROTO=none
IPADDR=192.168.2.203
PREFIX=24
GATEWAY=192.168.2.254
DNS1=192.168.2.254
DNS2=8.8.8.8
DNS3=8.8.4.4
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=no
NAME=eth0
UUID=41171a6f-bce1-44de-8a6e-cf5e782f8bd6
DEVICE=eth0
ONBOOT=yes

– Restart Network Service

systemctl restart network

– View dmesg for Newly Connected Hardware

dmesg
dmesg |grep sdb

– Mount USB or External Disk

mount /dev/sdb1 /media/usb

– Mount NTFS Partition

mount -t ntfs-3g /dev/sda2 /media/DiscoLocalC -o force

– Change System Timezone (Ubuntu)

dpkg-reconfigure tzdata

– TCP Traceroute to a Specific Port

traceroute -T -p 80 72.14.213.104

– Network Configuration in Ubuntu (interfaces file)

/etc/network/interfaces

– Example Static IP (Ubuntu)

auto eth0 iface eth0 inet static address 192.168.1.100 netmask 255.255.255.0 gateway 192.168.1.254 dns-nameservers 192.168.1.254

– Configure Static IP in Netplan (Ubuntu 18+)

sudo vim /etc/netplan/50-cloud-init.yaml

network: ethernets: enp0s3: addresses: [192.168.1.2/24] gateway4: 192.168.1.1 nameservers: addresses: [8.8.8.8,8.8.4.4] dhcp4: no version: 2

– Apply Netplan Config

sudo netplan apply

– Configure DHCP with Netplan

sudo vim /etc/netplan/50-cloud-init.yaml

network: ethernets: enp0s3: addresses: [] dhcp4: true version: 2

– Restart Networking (Ubuntu)

sudo ip addr flush eth0 && sudo systemctl restart networking.service

– Bring Network Interface Up

ip link set eth0 up

– Set IP Address

ip addr add 10.23.34.46/29 dev eth0

– Delete IP Address

ip addr del 10.23.34.46/29 dev eth0

– Set Default Route

ip route add default via 47.23.34.41 dev eth0

– View Routes (Route Table)

route -n

– View Routing Table (Alternative)

netstat -r

– Add Static Route

route add -net 200.66.81.64 netmask 255.255.255.192 dev eth0

– Bring Interface Down

ifdown eth0

– Bring Interface Up

ifup eth0

– Change MAC Address (MAC Spoofing)

ifdown eth0
ifconfig eth0 hw ether 00:01:02:03:04:05
ifup eth0

– Configure DNS Temporarily

echo "nameserver 192.168.16.254" >> /etc/resolv.conf

– Edit resolv.conf Manually

vi /etc/resolv.conf

– Flush DNS Cache

/etc/rc.d/init.d/nscd restart

– ARP: Add ARP Entry

arp -s 200.66.81.110 00:04:23:DC:D9:AC

– List ARP Table

arp -n

– ARP Neighbors

arp -an
ip neigh
sudo arp-scan -l

– Change Hostname (RedHat)

hostname nuevonombre

– Edit Hostname Permanently

Edit:

/etc/sysconfig/network

HOSTNAME=

– Change Hostname in Ubuntu

hostname nuevonombre

Edit:

/etc/hostname

– Mount Windows Shared Folder (CIFS)

mount -t cifs //servidor/carpetawindows -o username=luis.moreno,password=Misuperpassword /mnt/windows

– Older Systems (smbfs)

mount -t smbfs -o username=luis.moreno,password=Misuperpassword //servidor/carpetawindows /mnt/windows

– List Open Network Files

lsof -i -n

– Detect Link Speed (Ethernet)

ethtool eth0

SSH, SCP & Remote Access

– Enable SFTP and SCP Support (RedHat)

To use SFTP and SCP, ensure this line exists in /etc/ssh/sshd_config:

Subsystem sftp /usr/libexec/openssh/sftp-server

– Copy a Local File to a Remote Host

scp archivo-origen usuario@host:directorio/archivo-remoto

– Copy a File From a Remote Host to Local

scp usuario@host:directorio/archivo-origen archivo-destino

– SCP Using a Non‑Default Port (use -P)

scp -P 2222 usuario@host:/directorio/archivo-origen archivo-destino

– Copy an Entire Folder to a Remote Host (recursive)

scp -r directorio-origen usuario@host:directorio-destino

– Copy a Remote Folder to Local (with private key)

scp -r -i /Users/lmoreno/MyDropbox/EC2/EC2.private root@stream.com:/var/www/html/content/user_1/shoutcast198/* .

– Copy a Local File to Remote Using Private Key and Custom Port

scp -P 443 -i /Users/lmoreno/MyDropbox/EC2/Luisit0.private /Users/lmoreno/MyDropbox/EC2/EC2.private ubuntu@kolibers.com:/home/ubuntu

– Copy Local Datastore Folder to Remote Server

Example:

scp -r /vmfs/volumes/datastore/ root@192.168.16.1:/vmfs/volumes/
scp -Cpr /var/www/html/images/products lmorenor@192.168.16.5:/var/www/sites/www.mysitioweb.com/images/products

– Stop SSH Service

cd /etc/rc.d/init.d

Then:

sh sshd stop

Or:

service sshd stop

– Kill SSH Sessions (Find PIDs)

netstat -tanp

Look for ESTABLISHED SSH connections, then kill the PID:

kill 29460
Avoid using kill -9 unless absolutely necessary.

– Configure SSH Timeout (Disconnect Idle Users)

Edit /etc/ssh/sshd_config and add:

ClientAliveInterval 600
ClientAliveCountMax 3

This disconnects idle sessions after ~30 minutes.

– Change Default SSH Port (22 → custom)

Edit:

vi /etc/ssh/sshd_config

Locate:

#Port 22

Change to your custom port, e.g.:

Port 2995

Reload service:

service sshd reload

– If SSH Is Managed by xinetd

Edit:

vi /etc/xinetd.d/sshd

Change:

port = 22

Restart service afterward.

– SSH Through a Proxy Using corkscrew

Download corkscrew:

http://www.agroman.net/corkscrew/

Install:

cd Downloads
tar -xvzf corkscrew-x.y.tar.gz
cd corkscrew-x-y
./configure –host=apple
make
cp corkscrew $HOME/.ssh/

– Modify SSH Client Config to Use Proxy

Edit:

vim $HOME/.ssh/config

Add:

ProxyCommand $HOME/.ssh/corkscrew ip-del-proxy puerto %h %p

– Keep SSH Sessions Alive (Client Side)

Edit:

vim .ssh/config

Add:

ServerAliveInterval 60
ServerAliveCountMax 10

Client will try for ~10 minutes before disconnecting.

– Decompress .gz Files

gunzip /usr/share/wordlists/rockyou.txt.gz

– Decompress .7z Files

7za e hashcat-2.00.7z -ohashcat2

– Edit SSH Config File

vim $HOME/.ssh/config

– Send a Message to All Users (wall)

wall "Nada mas jugando"

– Find Your Current Terminal

tty

– Send a Message Directly to Another Terminal

echo "message" > /dev/pts/<number>

– Disable wall Message Reception

mesg n

– Send Email From Command Line

echo "20 minutos para la junta" | /bin/mail -s "Meeting" micorreo@miservidor.com

– Change SSH User Temporarily

su -s /bin/bash - apache

Processes & System Monitoring

– Show All Processes (including other users)

ps -al

– Show Processes With Their Process Tree and Associated Crontab

ps -auxfw

– Show Processes Using the Most CPU Resources

top

Press q to exit the top interface.

– View Open Files and Network Connections

lsof -i -n

To view files opened by a specific PID:

lsof -p 8295

– View System CPU Usage for a Specific Core

grep 'cpu1 ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage "%"}'

– CPU Usage for All Cores

grep 'cpu ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage "%"}'

– CPU1, CPU2, CPU3 Individually

grep 'cpu1 ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage "%"}'
grep 'cpu2 ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage "%"}'
grep 'cpu3 ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage "%"}'

– View Attempts of Failed Logins (btmp log)

last -f /var/log/btmp

– Secure btmp permissions (only root should read)

chown root:utmp /var/log/btmp && chmod 600 /var/log/btmp

– Check Which MTA (Mail Transfer Agent) Is Active

alternatives --config mta

– View History of Commands

history

– Show Commands 125 to 130 From History

fc -l 125 130

– View Command History With Timestamp

fc -li 100

– Set Date/Time Format for History

HISTTIMEFORMAT="%Y-%m-%d %T "

– Execute a Command From History (number n)

!n

– Clear Command History

history -c

Or manually edit:

~/.bash_history

– Send a Running Process to Background

ctrl+z

– Resume Background Process

bg

– Bring Background Job to Foreground

fg

– View Running Jobs

jobs

Package Management (RPM, YUM, DPKG, APT)

– Install an .rpm Package

rpm -ivh <nombre del archivo .rpm>

i = install
v = verbose
h = human-readable progress

– Example: Install Nessus

rpm -ivh nessus.rpm

– Check if a Package Is Installed

rpm -q "nombredelpaquete"

– List All Installed RPM Packages

rpm -qa

– Remove Packages (force if dependencies block uninstall)

rpm -e --nodeps php-pear-1.4.9-1.2
rpm -e php-5.1.6-1.2

– Find Which Package Provides a File

locate libdts.so.0

Then:

yum provides /etc/libdts.so.0

– Install a Package Using YUM

yum install <nombredelpaquete>

– Search for a Package With YUM

yum search <paquete>

– Get Detailed Info About a Package

yum info net-snmp-utils

– Remove Packages With YUM

yum remove <nombredelpaquete>

– Install a Package From a Specific Repository

yum --disablerepo=extras install paqueteA

To permanently exclude a package from a repo, add to .repo file:
exclude=paqueteA

– Install Unsigned Packages With YUM

Option 1: install via rpm
Option 2: set gpgcheck=0 in /etc/yum.conf

– Add EPEL Repository

wget http://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
rpm -ivh epel-release-latest-7.noarch.rpm

– Check Enabled Repositories

yum repolist

– Install Package From EPEL

yum --enablerepo=epel install zabbix

– Update the Entire System

yum update

– Install an Ubuntu/Debian Package (.deb)

dpkg -i nombredelarchivo.deb

If dependencies fail:

sudo apt-get install -f

Then run dpkg -i again.

– Example: Install TeamViewer

sudo dpkg -i teamviewer_linux.deb

When dependency errors appear, run:

sudo apt-get install -f

Then reinstall.

– Fix DPKG Overwrite Errors

sudo dpkg -i --force-overwrite <filename>

– Query Installed Packages in Ubuntu/Debian

yum list installed

(For rpm-based systems)

– Query Installed Packages With repoquery

repoquery -a --installed

– Install Packages on RedHat Using apt-rpm (legacy)

http://dag.wieers.com/rpm/FAQ.php#B

– Common Error With YUM Mirrors

[Errno 4] IOError: [Errno ftp error] 502 Command REST not allowed by policy.

Solution: download package manually into:

/var/cache/yum/updates/packages

Then run yum install again.

Firewall / IPTables

– IPTables Configuration File (RedHat/CentOS)

/etc/sysconfig/iptables

– Restart IPTables Service

service iptables restart

– Block a Specific Host

iptables -I INPUT -s 192.168.16.8 -j DROP

– Allow SSH on Port 22 Only From a Specific IP Range

iptables -A INPUT -p tcp --destination-port 22 -m iprange --src-range 192.168.16.100 -j ACCEPT

Use:
--src-range = e.g., 192.168.16.100-192.168.16.200

After adding rules, restart service:
service iptables restart

– List Active IPTables Rules

iptables -L

– Temporarily Allow an IP to Access FTP (port 21)

/sbin/iptables -I INPUT -s 200.52.66.116/32 -p tcp --dport 21 -j ACCEPT

– Remove the Rule After Testing

/sbin/iptables -t filter -D INPUT 1

– File With IPTables Custom Scripts

/etc/sysconfig/iptables

– View Open Ports Using netstat

netstat -pn -A inet

– View Listening Ports

netstat -lpn -A inet

– View All TCP Ports

netstat -lantp

Flags explained:

• -l listening
• -a all connections
• -n numeric ports
• -t TCP
• -p show PID/program
• -A inet show only TCP, UDP, raw, etc.

– View Listening Ports Using lsof

lsof -iTCP -sTCP:LISTEN

– View All Open Ports (TCP/UDP)

lsof -i

– Scan Localhost Ports Using nmap

nmap -sT -O localhost

– Identify Unknown Service by Port Number

Check /etc/services:

cat /etc/services | grep 834

If not listed, identify via netstat or lsof:

netstat -anp | grep 834
lsof -i | grep 834

– Find PID of a Running Service

pgrep sendmail

– Advanced grep + perl for nmap OS detection (unchanged from original)

cat nmap-os-db | perl -ne 'while(<>) { chomp;if (/^fingerprint\s+([^\#]+)/i) { if (defined($owin) and defined($cwin) and $owin ne $cwin) { print "$oname ($owin vs. $cwin)\n";} $oname=$1;undef($cwin);undef($owin);} elsif (/^T(4|6)\(.*W=([^%]+)/) { if ($1 eq 4){$owin=$2;} else { $cwin = $2; }}}' | sort -f

VNC, X11 & GUI Tools

– Set a VNC Password

vncpasswd

– Edit VNC Server Configuration (RedHat/CentOS)

File to edit:

/etc/sysconfig/vncservers

– Enable VNC Server at Boot

chkconfig vncserver on

– Start VNC Server

service vncserver start

– Configure Advanced VNC Session (custom desktop)

Edit:

/home/username/.vnc/xstartup

– GUI Firewall Configuration Tool

system-config-securitylevel

– View or Change X11 Display Settings

Main configuration file:

/etc/X11/xorg.conf

– Launch GNOME Desktop Environment

yum groupinstall "gnome"

– Repair GRUB Bootloader (after Windows overwrites it)

chroot /mnt/
grub-install /dev/sda

– Edit GRUB Menu Entries (rename OS options)

vi /boot/grub/menu.lst

Edit names like:

• “Other” → “Windows XP”
• “Other” → “Windows Vista”

– Remove GNOME Keyring Password Prompt

Delete:

.gnome2/keyrings/default.keyring

– Install rdesktop (Remote Desktop Client for Windows)

yum install rdesktop

Usage example: rdesktop -g 1024x768 windowssrv

– Hex Editor for Red Hat (GUI)

ghex

– View File in Hexadecimal (CLI)

hexdump -C <nombrearchivo>

– Change Display Resolution via X11 Config

Edit:

/etc/X11/xorg.conf

– Keyboard Shortcuts for iTerm2 Split Screen (macOS)

Horizontal split:

CMD + shift + D

Vertical split:

CMD + D

Crontab & Automation

– View Scheduled Cron Jobs

crontab -l

– Edit the Current User’s Crontab

crontab -e

– System Cron File (RedHat/CentOS)

Not recommended to edit directly, but located at:

`/var/spool/cron/root`

– System Cron File (Ubuntu/Debian)

/var/spool/cron/crontabs

– Cron Format Reference

*   *   *   *   *   Comando a ejecutar
-   -   -   -   -
|   |   |   |   |
|   |   |   |   +---- day of week (0–6; Sunday=0)
|   |   |   +-------- month (1–12)
|   |   +------------ day of month (1–31)
|   +---------------- hour (0–23)
+-------------------- minute (0–59)

– Example: Split a File Into Parts Based on Size

split -b 200M nombredearchivo

– Fix dpkg Update Error (force overwrite)

sudo dpkg -i --force-overwrite <filename>

– Example Error Message

error processing /var/cache/apt/archives/python-apport_2.0.1-0ubuntu9_all.deb

Solution (example):

sudo dpkg -i --force-overwrite /var/cache/apt/archives/python-problem-report_2.0.1-0ubuntu9_all.deb

Disk, Devices & System Boot

– Partition a Disk (create, delete, view partitions)

fdisk /dev/hda

– Create a Filesystem on a Partition (ext2 example)

mkfs -t ext2 -c /dev/hda1

-t = filesystem type
-c = check for bad sectors

– Show Disk List / Partitions

fdisk -l

– Mount a Floppy Disk (MS-DOS format)

mount -t msdos /dev/fd0 /floppy

– Mount CD-ROM

mount cd-rom
mount -t iso9660 /dev/cdrom /media/cdrom/

– Mount USB or External Disk

mount /dev/sdb1 /media/usb

– Mount a Partition on a Specific Directory

-- Mount /dev/hda2 /home

– Mount NTFS Partition (read/write)

mount -t ntfs-3g /dev/sda2 /media/DiscoLocalC -o force

– Check Kernel Boot Messages (useful when attaching new disks)

dmesg
dmesg | grep sdb

– Extract a .zip File (incorrect command in original, preserved)

tar -xzf [filename]

– List Files Inside a .zip

unzip -l filename

– Create an ISO Image From a CD-ROM

dd if=/dev/hdc of=/home/username/cdcopy.iso

Unmount CD before using dd:

umount /dev/hdc

– Alternative ISO Creation With mkisofs

mkisofs -r -v -J -l -o /opt/isocopymkiso.iso /media/PIAE_6/

– Burn ISO to Disc

cdrecord -v -pad speed=1 dev=ATAPI:1,0,0 src.iso

– Create a Device File

mknod /dev/fd1 b 2 1

– Repair GRUB Bootloader (after Windows installation)

chroot /mnt/
grub-install /dev/sda

– Edit GRUB Menu (rename or modify boot entries)

vi /boot/grub/menu.lst

Example: rename “Other” to “Windows XP”.

– Change Hostname Permanently (RedHat)

Edit:

/etc/sysconfig/network
Set:

HOSTNAME=<nuevonombre>

– Change Hostname (Ubuntu)

hostname nuevonombre

Edit file:

/etc/hostname

– Create File Filled With Zeros (useful for disk testing)

dd if=/dev/zero of=/test bs=1048576 count=1024

– View Disk Usage (human readable)

df -h

– View Folder Size

du -s /media/Nabucodonosor/data

– Summary of Subfolder Sizes (depth 1)

du --max-depth=1 -h

– Resize EC2 Volumes (reference only)

AWS docs:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-expand-volume.html

Miscellaneous Utilities

– Clear the Screen

clear

– Reset Terminal Settings

reset

– Print a Message to the Screen

echo

– Home Directory Shortcut

~

– Background Execution Operator

&

– Input/Output Redirection

< << > >>

– Pipe Operator

|

– Wildcards

?[]*

– Quoting Characters

' " \

– Command Substitution

`

– Grouping Commands

() {}

– Cancel Running Command

CTRL+C

– Remove Comments (#) from Config Files

cat /etc/squid/squid.conf | egrep -v "^\s*(#|$)"

– Convert DOS Newlines to Unix Format

tr -d '\r' < archivoDOS > archivoUNIX-LINUX

– Convert Using dos2unix

dos2unix <nombredelarchivo>

– Convert Filename Encodings (Windows → UTF-8)

convmv -f latin1 -t utf-8 -r --notest *

– Convert ANSI/ASCII Text Files to UTF-8

iconv -f 8859_1 -t utf-8 2lista.lst > lista.lst

– View File Type of Many Files (example for .mp3)

find ./ -name "*.mp3" -print0 | while read -d $'\0' file; do file "$file"; done > /opt/paso/mp3list.txt

– Check Laptop Temperature (ACPI)

acpi -t

– Create an Alias

alias ll='ls -lh --color=tty'

– Examples of Useful Aliases

alias logseg=‘less /var/log/secure’
alias tlogseg=‘tail -20 /var/log/secure’

– Make Alias Persistent (edit ~/.bashrc)

vi /home/<nombredeusuario>/.bashrc

– Remove an Alias

unalias ll

– Delete Matching Lines in a File Using sed

sed -e '/string-aqui/d' /opt/archivo-origen.csv > archivodestino.csv

– Remove CRLF From Windows Files

sed -i 's/\r$//' nombredelarchivo.txt

– Delete From a Pattern to End of File

sed -i '/mfa/,$d' ~/.aws/credentials

– Edit sudoers File

vi /etc/sudoers

To allow full sudo for user:

lmoreno ALL=(ALL)       ALL

– Extract Only Domain From Email List

cat lista.txt | cut -d"@" -f2 | sed 's/^/@/'

– Display Hexadecimal Output of a File

hexdump -C <nombrearchivo>

Keyboard Shortcuts

– Keyboard Shortcuts for Bash / Zsh Terminals

Movimiento del cursor en la consola

Edición de texto en consola

Navegación por el historial

Reutilización de comandos

Misceláneos

– iTerm2 (macOS) Split Window Shortcuts

Horizontal split:

CMD + shift + D

Vertical split:

CMD + D